Radians=diameter/distance. There are 57.296 degrees to a radian, in case this unit is unfamiliar. The barrel of a ballpoint pen cartridge (not the tip and the barrel of the pen) is 2mm diameter. Measure the distance from your eye to the pen in your hand. For me that is 24", 0.61 meters. Plugging 2mm (.002 m) into the equation above gives 3 milliradians. That is 3m at 1000m distance.
So how to use this? If you know the actual width or height of an object, distance in meters is actual size divided by apparent angle.
A human being is about 1.9 m tall, close enough to 2 m for our purposes. A sedan is about 1.3m high.passenger cars in the U.S. by law are less than 1.9m wide. While some older roads have 10 foot wide lanes, all newer ones are 4 m wide. Semis are 2.5m wide. Do the math.
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